Why do some deep-sea fish produce their own glow?

Answer: Chemical light helps them attract prey, signal, or hide

Chemical light helps them attract prey, signal, or hide ✓ — Light from chemical reactions can act as a lure, a signal, or camouflage, depending on the fish. Some fish make it themselves; others house light-producing bacteria. In the twilight zone, light from below can also help conceal a silhouette against faint light from above.

They store sunlight in their scales and release it later — A glowing fish is not simply releasing sunlight saved in its scales. Biological light comes from chemical reactions, which can occur in the fish's own tissues or in bacteria it hosts.

Pressure makes swallowed seawater give off light — Water pressure alone does not make swallowed seawater produce this glow. The light is generated biologically by chemical reactions, rather than by squeezing seawater inside the fish.